1. Technical Field
The present invention relates to a method for manufacturing a multilayer film optical element that is formed by laminating thin films consisting of substances having different refractive indices.
2. Background Art
An optical element that is provided with specified optical characteristics such as filtering by laminating thin films consisting of substances with different refractive indices and utilizing interference of light reflected at the boundaries of the thin films is known as a multilayer film optical element, and is used in an interference filter or the like.
Such a multilayer film optical element has a structure in which thin films of nonmetal optical substances with different refractive indices are successively superimposed on a substrate consisting of glass or the like, and is ordinarily formed by successive film formation of these nonmetal optical substances on a substrate consisting of glass or the like by means of vacuum evaporation.
In such an optical element, not only does the substrate consisting of glass or the like play no role in determining the optical characteristics, but it also absorbs light, so that it is necessary that the substrate be as thin as possible. Accordingly, such substrates have conventionally been polished following the multilayer film formation to keep the thickness down to approximately several tens of microns.
However, it is extremely difficult to polish a substrate consisting of glass or the like to a thickness of approximately several tens of microns, and there is a problem in that the yield is lowered due to damage to the glass during polishing.
A multilayer film optical thin film that does not have a substrate as described, for example, in Japanese Patent Application Kokai No. H3-196001, has been publicly known as an optical element that solves such a problem.
Such a multilayer film optical thin film is manufactured as follows: For example, aluminum is deposited on a glass substrate, and silicon oxide thin films and titanium oxide thin films are alternately formed on top of this by means of ion sputtering. If the aluminum is dissolved by an aluminum etching liquid at the completion of the film formation, the glass substrate and the multilayer optical thin film are separated, so that a multilayer optical thin film having no glass substrate can be obtained.
However, as a result of the experiments conducted by the inventor, it was found that there are major problems in the method for manufacturing a multilayer optical thin film of the type described in Japanese Patent Application Kokai No. H3-196001. That is to say, in cases where a multilayer optical thin film is formed by ion sputtering or the like, projections and indentations are produced at the interface between the aluminum and multilayer optical thin film; as a result, the multilayer optical thin film is clouded, leading to a significant drop in the transmissivity of light, so that such an optical thin film cannot withstand practical use. Aside from this, it was also found that aluminum does not adequately play a role as a carrier, so that the problem of incomplete separation of the glass substrate and multilayer optical thin film was encountered.